In the manufacture of e.g. hard board, the pressure in the hot-pressing step is usually high and a dry content of about 50 per cent in the wet sheet often is required after the final mechanical squeezing out in order that the remaining water shall be removable by evaporation. Such high dry content prior to the evaporation can not be brought about in a conventional wet-forming machine, which usually does not render a higher dry content than 30 to 40 %. An increase of the dry content from 30 to 55 % can be brought about only in high-pressure equipment, e.g. flat presses, operated with necessary pressure, e.g. 50 to 75 kgs/cm.sup.2. Therefore, this dewatering operation can be carried out as a separate pressing step prior to the hot pressing operation proper, or possibly, in a pressing portion mounted directly to the sheet forming machine. However, it is quite possible to effect the last mechanical squeezing out of water in the hot press if the drained-off water is collected and returned to the backwater system. Due to the high temperature in the hot press, this involves some inconveniences by generation of sirupy or resinous coverings on the fibre sheet or web.
As pointed out above, the entering wood fibres must have a considerably higher dry content than the wet sheet prior to the final drying step, if it shall be possible to form the wet sheet in a wholly closed backwater system. Therefore, in the manufacture of hard board, the dry content of the fibres prior to the suspension thereof must be brought up to 60-75%. A high dry content of the fibre material renders possible to add certain quantities of fresh water e.g. from pressurized waterproof chests and the like, without causing any excess of backwater.
When starting from very dry wood having a dry content of 70-80 %, which can have been reached by storing or artificial drying of the wood material, it is possible to produce a fibre pulp having sufficient high dry content; normally, however, the wood has so high a moisture content or so much water must be supplied in the defibrating step that the dry content of fibre material becomes too low.
Of course, the fibre pulp can be dried in all known and suitable ways, but in order to ensure a constant and sufficiently high dry content of the fibre material, it is suitable to produce the pulp in accordance with the disclosure in the French patent No. 7210434, according to which the wood is defibrated in an atmosphere of saturated steam and pulp discharged from this treatment is dried continously to a dry content which can be varied within a wide range from e.g. 40 to 90 % and usually 60 to 75 %, and thereupon is suspended in circulating backwater, under addition, if desired, of some fresh water, and then formed and pressed or solely dried. By the drying of the fibre material to suitable dry content, fibreboard or slabs can be manufactured in a wholly closed backwater system without discharge of process water. In this way a method is obtained, against which no objections from the view point of perservation of nature can be raised.
Defibration under pressure and increased temperature is effected usually within a temperature range from 160.degree. to 170.degree. C. Under these conditions, the quantity of released organic material becomes relatively high, viz. 7 to 10 %, and its content in the circulating backwater is increased gradually. Therefore, it is recommended to operate at a lower temperature within a range from 130.degree. to 150.degree. C and a short preheating time, in which way the release of organic material can be lowered and limited to 4 to 5 %. Thereby, a backwater is obtained which has a lower concentration of organic substances dissolved therein. Nevertheless, it must be calculated with that the backwater will have a relatively high content of hydrolyzed hemicellulose, dextrines, low-molecular lignine and also resins. These various substances cause precipitations in the backwater and can produce spots on th finished board with a classing down of the quality of the board as an unwelcome consequence, and in addition increase the danger of fire in subsequent heat treatment of the pressed board or slab.